1. Field of the Invention
The present invention relates to shaped heat hardened base metal alloy articles which are initially coated with a layer of noble metal especially gold which is, in turn, diffused into the body of the base metal alloy. The invention is particularly concerned with producing shaped metal articles comprised of a base metal of copper alloy over which lies a covering of gold or gold alloy. Shaped metal articles according to the present invention are utilized where the ultimate article being manufactured is one which requires good machineability properties, strength, durability and resistance to corrosion and discoloring. This invention is particularly concerned with the formation of shaped metal articles of the type described which serve a decorative function and which can be polished to a high degree of sheen such as watch bezels.
It is important that the article manufactured be relatively hard, resistant to dents, scratches and the like and have a durable and attractive coating of a noble metal which can be polished to a high sheen and is resistant to pits, scratches, discoloring and corrosion.
2. Prior Art
Various techniques have been employed in the past in the manufacture of articles from heat hardenable copper alloys which are diffusion coated with a noble metal. It has generally been found that it is unsuitable to coat the copper alloy prior to the shaping and finishing operation because of the physical changes which occur in the properties of the underlying body and the coating during the shaping and finishing operation. The thickness of the plated layer was often changed in forming steps after coating so that it varied in an irregular manner as well as disturbed the smooth finish of the coating metal producing a rough, uneven surface which was difficult to finish to an aesthetically acceptable degree. Excessive polishing and the like also diminished the thickness of the finished layer to an unacceptable degree.
In instances where the article had been plated before shaping and other fabrication steps, any cutting and the like would expose unplated portions to corrosion as well as ruining the aesthetic appearance of the article.
Various alternative approaches have been attempted to overcome these problems. One obvious approach is to form the shaped body of the base metal or alloy and apply the noble metal coating only after the shaping of the article has been completed. The adherence of the noble metal coating to the shaped body is then assured by diffusion heating which at first was carried out for short periods of time at relatively high temperatures often within the softening range of the base metal or base metal alloy. In order to prevent the deformation of the article during high temperature diffusion heating, the treatment was carried out in a hot salt melt which unfortunately, however, often adversely effected the mechanical properties of the product. In cases where the noble metal was merely plated on to the underlying shaped base metal article and not diffused into the shaped base metal article by subsequent diffusion heat treatment, the coating lacked sufficient adherence to provide the desired durability and corrosion resistance.
In U.S. Pat. No. 3,157,539 Dreher, a process is disclosed for making shaped metallic bodies having a noble metal coating wherein the underlying metal or alloy is one which is heat hardenable by shaping the object from heat hardenable copper alloy in its unhardened condition, applying a coating of noble metal to the shaped body of the base metal which noble metal is adapted to be firmly joined to the shaped body of the copper alloy by diffusion heat treatment. The patentee indicates that the underlying base metal is hardenable within a first temperature range, that the heat diffusion process takes place within a higher second temperature range. The first and second temperature ranges partially overlap each other to define a third temperature range in which the formed and coated metal body is finally heated to simultaneously harden the underlying base copper alloy and diffuse the noble metal coating into the formed base metal. The process disclosed by the patentee comprises in effect a compromise between hardening the underlying metal at the preferred temperature range and simply adopting a single temperature range to simultaneously carry out both processes at the supposed third temperature range which is ideal neither for hardening nor diffusing. Thus, the final shaped and coated metal body has neither the best mechanical properties nor is the diffused coating as firmly adhered and uniformly diffused as desired. As indicated in the patent, the temperature range at which the hardening and diffusing is simultaneously carried out ranges from 350.degree.C to 600.degree.C with a temperature of apparently 500.degree.C being preferred. The base metal utilized in this patent is a copper or copper alloyed with beryllium or with silicon and manganese, or with silicon and nickel. The plating metal is gold or a gold alloy.
An additional difficulty which is encountered using some of the alloys previously employed in forming shaped metal articles mentioned is the difficulty in machining and shaping the article. This is believed due to the fact that most of the previous alloys employed in this type of manufacturing process have excluded free machining additives such as lead, selenium, tellurium or others.
A recent approach in this area to provide a shaped metal article of increased hardness and corrosion resistance has employed as the underlying metal a copper alloy which is a bronze comprised of copper, aluminum and nickel from which tin, zinc and lead have been excluded.
In one recent development, a bronze alloy of aluminum, nickel and copper has been electrically plated with about 4 microns of gold following machining. The machined workpiece was a watch bezel. After plating, the plated piece was simultaneously hardened and the gold layer diffused in a heat treatment.
It was thought necessary in this process to coat the gold diffused coating with a final coating of a hard metal selected from chromium, rhodium, ruthenium and the like to a thickness of approximately 20 microns because the diffused gold layer was not durable enough.